As development of rotor spinning machines progresses, the goal is not only to improve the quality of the yarns produced, but above all to increase production capacity. A key factor in increasing production capacity is the rotary speed of the spinning rotor. For this reason, varied kinds of drives and bearings for spinning rotors have been developed, in order to reach rotary speeds of markedly over 100,000 rpm. Reducing the rotor diameter and mass and lowering friction losses enables not only greater rotary speed but also reduced energy consumption when driven.
In this respect, a shaftless spinning rotor, which is embodied as the rotor of an axial field motor, can be considered especially advantageous by providing a combined magnetic and gas bearing which assures relatively low friction losses.
A shaftless open-end spinning rotor of the above-described type having a combined magnetic and gas bearing is known from International PCT Patent Reference WO 92/01096, which discloses a rotor having a bearing face, remote from the spinning chamber of the spinning rotor, and means for conducting the magnetic flux for the driving and guiding magnetic field. By means of the guiding magnetic field, the rotational axis of the open-end spinning rotor is to be rigidly defined and maintained during rotation. However, it has been found impossible to achieve significant suppression of impermissible vibratory, wobbling and oscillating motions that occur particularly in critical rpm ranges.
Permanent magnets located opposite each other in the rotor and the stator and having facing magnetic poles of reversed polarity are provided for generating the magnetic guide field. However, irregularities in the dimensions and in the magnetization of these magnets often occur in the stator and the rotor, which lead to deviations in magnetic induction and inhomogeneities in magnetic flux distribution and thereby can cause a radial mismatch between the magnetic axis of symmetry and the axis of rotation through the center of gravity of the rotor. Further, it is also possible that opposite actions of two concentric magnetic systems in the rotor and stator can cause radial oscillations of the rotor and output losses. Further disadvantages of the known construction lie in the production expense caused by the necessity of extremely accurate dimensioning and exact positioning required of the magnets in the stator and rotor, typically performed by means of an elaborate gluing process. Furthermore, the disposition of permanent magnets in the rotor causes problems in that on the one hand the bearing face of the rotor should have very little roughness, but on the other hand, when permanent magnets are disposed in the bearing face, mechanical finishing of this surface in the form of grinding and polishing is made very difficult without mechanical destruction or demagnetization of the magnets because of the resulting effects of temperature.